1. Field of the Invention
The present invention relates to the method of intrusion detection for detecting the presence and location of objects in space and, more particularly, to the method of operating an active infrared system for actively locating objects in space by emitting a beam of infrared radiation into a field of view and measuring the distance from the device at which the beam is reflected by the field of view during a selected time period. The distance of the reflection is then compared to a reference signal, with an output signal being generated if the detected distance differs from the reference distance in a preselected manner.
2. Background Art
In recent years many forms of intrusion detection devices, or surveillance systems, have been developed to monitor an area or space, to protect against the entry of unauthorized personnel into that area or space, and to provide an alarm signal when such entry occurs. A variety of technologies have been applied to such intrusion detection systems in an attempt to obtain a satisfactory device that provides the requisite sensitivity to intrusion into the protected space without generating distracting, time-consuming, and costly false alarms. Such false alarms can result from changing environmental conditions, wind-blown debris, or the intrusion into the protected space of birds or other small animals. Among the technologies employed for intrusion detection systems of the prior art are those based on sonic or ultrasonic/acoustical detectors, photoelectric break-beam devices, passive infrared detectors, video systems, and radar or microwave-based systems.
The sonic, ultrasonic or acoustical devices are illustrated in U.S. Pat. Nos. 4,499,564, 4,382,291, 4,229,811 and 4,639,902. In the devices disclosed in these patents the intrusion detection systems utilize an acoustical signal, either sonic or ultrasonic, which is transmitted into the space to be protected. The acoustical signal is reflected off of objects in the space or the walls forming the perimeter of the space and is collected by an acoustical receiver. The return signal represents the total reflected energy pattern for that space. A change in the signal received indicates some change in the space protected; however, these systems do not provide any means of identifying where, either directionally or distance-wise, in the protected space that the change has occurred. Thus, the only information derivable from such systems is whether or not such a change has occurred which then requires some form of follow-up by the security force. An additional limitation of systems of this type is that they are generally unacceptable in anything but a closed environment since they are subject to false alarms from naturally occurring sound changes such as generated by wind, thunder, or other naturally occurring sounds in an open environment.
The photoelectric break-beam devices are illustrated in U.S. Pat. Nos. 3,875,403, 4,239,961, 4,310,756, 4,384,280 and 4,514,625. In the devices disclosed, the intrusion detection system uses an active photo-beam projected into the area under surveillance. A detector sees the continuous beam at the opposite end of the detection zone. If the photo-beam is broken by an intruder, then an alarm is sounded. This type of system does not give any information above the distance of the intruder from the detector device. This system also requires two head units with the protection zone between them. This leads to a more complex installation than if only one unit is required.
Passive infrared detection technology is illustrated in U.S. Pat. Nos. 3,476,946, 3,476,947, 3,476,948 and 3,475,608. With systems such as these, changes in the infrared content of the light received by the device from the area under control is monitored and an alarm signal is generated if the infrared content changes. This is based on the presumption that the infrared content of the light will be affected by intruders, particularly individuals, entering into the controlled space. However, it has been found that such infrared detectors are falsely triggered by normal changes in the infrared content of the light in a space due to ordinary changes in the sun as well as the effects of clouds passing over the sun. Still further, such systems do not provide distance or direction information and thus require follow-up by security staff to determine the true nature of the cause that triggered the alarm.
The video based intrusion detection systems utilize a video camera to view an area under protection and are illustrated in U.S. Pat. Nos. 3,823,261; 3,932,703 and 4,408,224. Typically, the video signal is digitized and stored in a memory. Thereafter, the video signal is compared with a reference signal stored in the memory and, when a difference is detected, an alarm is sounded. These systems use changes in scene illumination to determine an alarm condition rather than changes in object distances and therefore, unless the space to be observed and protected is carefully controlled and isolated from changes in environmental illumination, such changes will result in false alarms. As a result, such a system is less than satisfactory for exterior spaces. Furthermore, the amount of data that is necessarily stored to obtain reasonable resolution of the image of the space being protected requires a significant quantity of expensive computer memory.
Systems employing radar or other microwave technology are illustrated in U.S. Pat. No. 4,197,537. In this particular system a single microwave signal source is used to bathe the space with microwave energy. A receiver detects the return signal reflected from the space being protected which can be compared with a reference signal to detect an intrusion thereinto. This particular system is unable to identify the precise location of the intruder. While other radar/microwave-based systems can provide such information, their cost is at least an order of magnitude greater than the cost of the other systems described above as well as that of the present invention.
Thus, a method of intrusion detection which is both economical, sufficiently sensitive and yet relatively immune to false alarms would be very attractive to the security industry .